Chance that a new monster is generated. Currently depends on the
number of monsters already present, and on the level. In the future,
the strength of the character and the strength of the monsters present
could further influence the chance, and the chance could also affect
which monster is generated. How many and which monsters are generated
will also depend on the cave kind used to build the level.

The Actor type

Actor properties that are changing throughout the game.
If they are dublets of properties from ActorKind,
they are usually modified temporarily, but tend to return
to the original value from ActorKind over time. E.g., HP.